Remove redundant harbor-postgres override

sylva-projects/sylva-core!8573 (merged) removed the harbor-postgres unit. We still have the override in this repository, which will lead to failures, as observed in https://gitlab.com/sylva-projects/sylva-core/-/jobs/16539504883.

Since the harbor-postgres unit is using

enabled_conditions:
      - '{{ tuple . "harbor" | include "unit-enabled" }}'

and we still have the harbor: enabled override kept intact, we won't affect the functionality of the misc pipelines by removing the harbor-postgres override. This MR is implementing this in order to fix the misc pipelines for sylva-core main.